Activation of cellular oncogenes by environmental carcinogens may represent a critical process in the mechanism of carcinogenesis. Animal tumor models using defined carcinogenic protocols are necessary tools to understand the precise molecular relationships between the effects of carcinogens and the biological end point of cancer. In this proposal, rat skin tumors induced by ionizing radiation will be examined for activation of the ras family of transforming oncogenes as well as activating alterations of the myc oncogene. The frequency and nature of point mutations, gene amplification, chromosomal translocations, and enhanced transcriptional activity of active oncogenes will be compared in a large panel of tumors of various histologic type. Preliminary results have indicated that both K-ras and myc are activated in some of these tumors. Active oncogenes will be examined on a molecular level by Southern and Northern analysis, gene cloning and sequencing, and in situ hybridization. Studies on the temporal aspects of oncogene activation as a function of radiation exposure will be done using the C3H10T 1/2 cell culture system, as well as rat skin in vivo. A new clonal cell hybridization technique will be used to screen monoclonal populations of irradiated cells for gene amplification and gene expression at sequential times after irradiation. One hypothesis to be addressed by the proposed research is that radiation effects on target DNA leads directly to activation of myc. A more general issue that will be studied in this model system relates to the significance of multiple oncogene activation in primary epithelial tumors. A greater understanding of the molecular mechanisms by which environmental carcinogens such as radiation produce neoplastic transformation will be relevant to the design of strategies for detection, prevention and control of cancer.